This invention relates to an optical waveguide apparatus for use in a wavelength division multiplex (WDM) optical communication system as well as a method of producing the same. More specifically, this invention relates to a planar optical waveguide device for implementing a wavelength selecting function, such as an arrayed waveguide grating (AWG) used as an optical signal multiplexer or demultiplexer, as well as a method of producing the same.
In recent years, a wavelength division multiplex (WDM) transmission system becomes widely used in optical transmission. In the WDM transmission system, a number of signals different in wavelength from one another are multiplexed and transmitted through a single optical fiber. As a greater number of signals are multiplexed, a transmission capacity is increased. Most recently, 100 or more signals different in wavelength are multiplexed. As a consequence, a separation or spacing between different wavelengths is narrowed. For example, in a system of a 100 GHz grid, the spacing between two adjacent wavelengths must be equal to 0.8 nm. The WDM transmission system is initially used in a long-distance network but is growing wider applications covering a periphery of a terminal.
In the above-mentioned WDM transmission system, a device having a wavelength selecting function of selecting a particular signal among a number of signals different in wavelength is essential and indispensable. Such wavelength selecting function is provided by a planar optical waveguide device as an integrated device.
As an example of the planar optical waveguide device having such a wavelength selecting function, an arrayed waveguide grating (AWG) is disclosed in Japanese Patent No. 2599786 (JP 2599786 C). The arrayed waveguide grating is used as an optical multiplexer/demultiplexer. Referring to FIG. 1, a waveguide pattern of silica-based glass is formed on a substrate 1. The waveguide pattern includes at least one optical input waveguide 2, an input-side slab waveguide 3 as a first slab waveguide, a plurality of patterned or arrayed waveguides (channel waveguides) 4 different in length from one another, an output-side slab waveguide 5 as a second slab waveguide, and at least one optical output waveguide 6 (in the illustrated example, a plurality of optical output waveguides 6 are shown) which are successively connected in this order. A combination of the arrayed waveguides 4 forms a diffraction grating 14 so that the arrayed waveguide grating is provided. For simplicity of illustration, only a small number of waveguides are shown in FIG. 1. In an actual device, the arrayed waveguides are equal in number to about 100. The number of the optical output waveguides corresponds to the number of output channels.
The optical input waveguide 2 is connected to an optical fiber (not shown) so as to introduce a wavelength-multiplexed light beam. The light beam introduced through the optical input waveguide 2 into the input-side slab waveguide 3 is spread due to a diffracting effect of the input-side slab waveguide 3 to be incident to the respective arrayed waveguides 4 as split beams which propagate through the respective arrayed waveguides 4. The split beams propagating through the respective arrayed waveguides 4 reach the output-side slab waveguide 5. The split beams reaching the output-side slab waveguide 5 are condensed or focused as a focused beam which propagates into the optical output waveguides 6 to be outputted therefrom.
In the arrayed waveguide grating described above, the arrayed waveguides 4 are different in length from one another. Therefore, after the split beams delivered from the input-side slab waveguide 3 propagate through the respective arrayed waveguides 4, the split beams are shifted or differed in phase from one another. Depending upon the magnitude (quantity) of the phase shift or difference, the wavefront of the focused beam is tilted. A focusing position is determined by the tilting angle of the wavefront of the focused beam. Therefore, by forming the optical output waveguides 6 at that position, output light beams different in wavelength from one another can be produced from the optical output waveguides 6 corresponding to the different wavelengths, respectively.
In the arrayed waveguide grating, the diffraction grating 14 has a wavelength resolution proportional to a difference (xcex94L) in length between the arrayed waveguides 4 forming the diffraction grating 14. Therefore, by designing the diffraction grating 14 with a greater value of xcex94L, it is possible to carry out optical multiplexing and demultiplexing for multiple light beams at a narrower wavelength spacing.
However, in the above-mentioned arrayed waveguide grating, the patterened waveguides 4 are different in length from one another. This means that variations in length (optical path length) of the arrayed waveguides 4 in response to the variation in device temperature are different from one another. Therefore, in response to the variation in device temperature, filtered wavelengths, i.e., wavelengths demultiplexed by the arrayed waveguides 4 are greatly changed.
In order to solve the above-mentioned problem, it is proposed to introduce a temperature control mechanism into the optical multiplexer/demultiplexer. The temperature control mechanism comprises a Peltier device for cooling and a temperature control circuit and carries out temperature control of the arrayed waveguide grating so that the temperature variation itself is eliminated. However, introduction of such a temperature control mechanism results in an increase in size of the apparatus, an increase in cost, and an increase in power consumption.
As another approach without using the Peltier device, proposal is made of a method which will hereinafter be described in conjunction with FIG. 1. In order to cancel the temperature dependence of the arrayed waveguides 4 of the arrayed waveguide grating, a trapezoidal groove is formed across the arrayed waveguides 4, as depicted by a broken line in FIG. 1. The arrayed waveguides 4 comprise silica-based glass cores having a positive temperature coefficient of refractive index. A temperature compensating part 9 is formed by filling the trapezoidal groove with silicone resin having a negative temperature coefficient of refractive index.
By canceling the variation in optical path length due to the temperature-dependent variation in refractive index of each arrayed waveguide 4, it is possible to remove the temperature dependence of the arrayed waveguide grating (see xe2x80x9cAthermal silica-based arrayed-waveguide grating (AWG) multiplexerxe2x80x9d, ECOC ""97 Technical Digest, pp. 33-36, 1997). In this approach, the temperature dependence of the transmission wavelength of the arrayed waveguide grating is reduced to a small value equal to 0.001 nm/xc2x0 C. or less.
However, with the above-mentioned structure, optical mismatch is caused between the arrayed waveguides and the temperature compensating part filled with silicone resin. Furthermore, it is difficult to form a cladding layer on the temperature compensating part 9 of a trapezoidal shape formed in the arrayed waveguide grating. This brings about occurrence of excessive loss in a region of the temperature compensating part 9 of a trapezoidal shape. As a consequence, the optical transmission loss characteristic of the arrayed waveguide grating as a whole device is degraded.
As a still another approach, EP 0849231 A1 discloses a method of improving the temperature characteristic by selecting a material of the waveguide. This method aims to improve the temperature characteristic of the device resulting from the difference in temperature dependence between the waveguides different in material. By exactly matching the optical path length temperature-dependent variation rate of two waveguides, the temperature characteristic of the wavelength control function is improved.
However, the above-mentioned method is not applicable to a device such that the waveguides are made of a same material and the wavelength control characteristic is achieved by the difference in physical length between the waveguides. Furthermore, even if the temperature characteristics of the individual waveguides are rendered identical by the use of the waveguides of the same material, a desired wavelength control characteristic can not be achieved unless the temperature characteristics of the individual waveguides are sufficiently low.
FIG. 2 shows a production process of a ridged optical waveguide widely used. In a first step, a core layer 4a is formed on the substrate 1. In a second step, the core layer 4a is patterned by a lithography or the like to form a plurality of cores 4. In a third step, an upper cladding layer 10 is formed to cover the cores 4. Thus, the cores 4 are surrounded by the upper cladding layer 10 and the substrate 1. Therefore, the substrate 1 may be called a lower cladding layer. A combination of the upper and the lower cladding layers may be collectively referred to as a cladding surrounding the cores 4.
Herein, glass thin films as the core layer 4a and the upper cladding layer 10 are formed by flame hydrolysis deposition. In case where the thin film is formed by the flame hydrolysis deposition, heat treatment is required after the thin film is formed. This is because a resultant deposit (called a soot) obtained by the flame hydrolysis deposition is low in density and must be increased in density in order to achieve excellent optical characteristics and low propagation loss. Therefore, in case where a planar waveguide device of a multilayer structure is formed by the above-mentioned thin film forming technique (flame hydrolysis deposition), a structure formed in a later step of the production process must have a glass transition point lower than that of a structure formed in an earlier step. Specifically, a glass of the upper cladding layer must have a glass transition point lower than those of a core glass and the substrate. As a consequence, the upper cladding layer 10 and the substrate 1 as the lower cladding layer are made of different glass materials. In this case, the upper and the lower cladding layers are different in coefficient of thermal expansion. This results in undesired stress applied to the core 4 as the optical waveguide.
As another production process, it is possible to form the upper cladding layer 10 by the use of chemical vapor deposition (CVD). In this case, the upper cladding layer 10 can be formed by the material same as that of the substrate 1 as the lower cladding layer. However, cracks or voids are locally formed between the upper cladding layer 10 and the cores 4. Thus, the upper cladding layer 10 and the cores 4 are not always kept in mechanical tight contact with each other.
After the thin film is formed, the heat treatment is required to remove the cracks or the voids formed in the upper cladding layer 10. In case where the heat treatment is carried out under such conditions that the cracks or the voids can be removed, not only the lower cladding layer formed by the same material is deformed but also the cores 4 typically lower in softening point are deformed. Therefore, it is difficult to achieve mechanical bond between the cladding and the core by the heat treatment.
As described above, the conventional arrayed waveguide gratings have various disadvantages. Furthermore, the existing methods of producing an optical waveguide apparatus such as the arrayed waveguide grating have several disadvantages also. These disadvantages are not restricted to the arrayed waveguide grating but apply to other typical optical waveguide apparatuses.
It is therefore an object of this invention to provide an optical waveguide apparatus which is practically free from temperature-dependent variation of an optical path length, low in optical transmission loss, small in size, and low in cost.
It is another object of this invention to provide a method of producing the optical waveguide apparatus mentioned above.
According to an aspect of this invention, there is provided an optical waveguide apparatus comprising a substrate, a core formed in a recessed portion of the substrate, and an upper cladding layer formed on the substrate, the core being surrounded by a cladding comprising the substrate as a lower cladding layer and the upper cladding layer and smaller in refractive index than the core;
the core and the cladding being integrally coupled to each other in a manner such that temperature-dependent expansion or contraction is performed substantially in accordance with the characteristic of the cladding;
the core and the cladding being made of materials selected so that the variation in optical path length of the core according to the temperature-dependent expansion or contraction of the cladding is canceled by the variation in optical path length according to temperature-dependent variation in refractive index of the core.
Preferably, each of the core and the cladding is made of silica-based glass. The core is made of a material having a negative temperature coefficient of refractive index.
Preferably, each of the substrate and the upper cladding layer is made of Ti-doped SiO2 or F-doped SiO2.
Preferably, the cladding is made of Ti-doped SiO2 while the core to be combined therewith is made of a material selected from those having a smaller temperature coefficient of refractive index as compared with the case where the cladding is made of SiO2.
Preferably, the cladding is made of F-doped SiO2 while the core to be combined therewith is made of a material selected from those having a greater temperature coefficient of refractive index as compared with the case where the cladding is made of SiO2.
Preferably, the core is made of a material containing B2O3.
Preferably, the core is made of a material selected from a SiO2xe2x80x94GeO2xe2x80x94B2O3 glass, a SiO2xe2x80x94TiO2xe2x80x94B2O3 glass, and a SiO2xe2x80x94GeO2xe2x80x94B2O3xe2x80x94P2O5 glass.
Preferably, the contents of GeO2 and B2O3 have a ratio of 2:1 to 3:1.
Preferably, the cladding is made of a material selected from those having a refractive index lower than that of the material of the core;
the cladding material and the core material being combined so that the temperature coefficient of refractive index of the core material is different in sign from and equal in magnitude (absolute value) to the coefficient of thermal expansion of the cladding material.
According to another aspect of this invention, there is provided a method of producing an optical waveguide apparatus, comprising the steps of:
forming a recessed portion having a predetermined pattern on a surface of a substrate;
forming a core in the recessed portion of the substrate by the use of a material having a refractive index higher than that of the substrate, a temperature coefficient of refractive index which has a value reverse in plus/minus sign to a coefficient of thermal expansion of the substrate, and a glass transition point lower than that of the substrate; and
heat-treating the substrate with the core formed in the recessed portion at a temperature higher than the glass transition point of the material of the core formed in the recessed portion of the substrate and lower than the glass transition point of the substrate.
Preferably, the predetermined pattern corresponds to an optical waveguide pattern forming an arrayed waveguide grating.
Preferably, the heat treating step is carried out at a temperature higher than the glass transition point of a core glass and lower than the glass transition point of SiO2 as a cladding glass.
Preferably, the step of forming the core is followed by the step of forming an upper cladding layer to cover an upper surface of the substrate and an upper surface of the core formed in the recessed portion of the substrate;
the step of forming the upper cladding layer being followed by the step of heat-treating the substrate.
Preferably, the substrate and a glass plate to become the upper cladding layer are put into optical contact at normal temperature and then heat treated at a temperature higher than the glass transition point of the core material and lower than the glass transition point of the cladding material.
According to the above-mentioned aspects of this invention, it is possible to provide the optical waveguide apparatus capable of principally eliminating the temperature-dependent variation in optical path length. Specifically, the temperature-dependent variation of the optical path length of the optical waveguide apparatus is theoretically given by the following equation (1):
d(nL)/dT=nL{1/n(dn/dT)+1/L(dL/dT)}xe2x80x83xe2x80x83(1)
where n represents a refractive index of the optical waveguide, L, a physical length of the optical waveguide, T, a temperature. Considering that most of energy of a propagating light beam concentrates to the interior of the core, n is substantially represented by the refractive index of a core material. From the above-mentioned logic, the variation rate of the physical length L corresponds to the coefficient of thermal expansion of the core. If the core is mechanically constrained by the substrate as the lower cladding layer and the upper cladding layer, the coefficient of thermal expansion of the core is substantially represented by the coefficient of thermal expansion of a cladding material because the upper and the lower cladding layers have a volume much greater than that of the core.
From the equation (1), it is understood that the temperature-dependent variation in optical path length can be eliminated when the term (1/n(dn/dT)) corresponding to the temperature coefficient of refractive index of the core material is reverse in plus/minus sign and equal in value (absolute value) to the term (1/L(dL/dT)) corresponding to the coefficient of thermal expansion of the cladding material. Since the coefficient of thermal expansion of SiO2 widely used as the cladding material, the temperature coefficient of refractive index of the core material must have a negative value.
In case where the above-mentioned condition is implemented, for example, by a planar optical waveguide, it is desired that a core is mechanically tightly constrained by the cladding comprising the upper and the lower cladding layers and surrounding the core. Therefore, in the above-mentioned aspects of this invention, it is preferable that the glass substrate as the lower cladding layer is provided with the recessed portion in which the core is buried and that the upper cladding layer is formed by the material same as that of the lower cladding layer. In order to perform the heat treatment to achieve the mechanical tight contact between the core and the cladding, it is essential that the glass transition point of the core material is sufficiently lower than that of the cladding. In the device of the above-mentioned structure, the shape of the core is retained by the cladding having a higher glass transition point. Therefore, after the core is formed, the shape of the core is not deformed by the heat treatment at a high temperature. Furthermore, during the heat treatment after the upper cladding layer is formed, the core is not deformed. Thus, it is possible to perform the heat treatment at a relatively high temperature to achieve the mechanical tight contact between the core and the cladding.
According to the above-mentioned aspects, it is possible to practically eliminate the temperature-dependent variation in optical path length without additionally using temperature stabilizing means, such as a Peltier device, or without inserting a material different in temperature coefficient of refractive index in the middle of an optical path. Thus, according to the above-mentioned aspects, it is possible to easily obtain an optical waveguide apparatus which is practically free from temperature-dependent variation in optical path length, small in optical transmission loss, small in size, and low in cost.